Accurate measurement of gas flow is necessary in the operation or control of many processes. It is, for example, customary to sample the atmosphere of a workplace or hostile environment to detect the presence of a specific constituent or contaminant and its concentration. This is done by drawing a known volume of air through an air sampling system containing one or more filters or sorbent tubes which may be directly or subsequently analyzed to quantify the leval of the constituent or contaminant in the air sample. The accuracy in calculating the concentration of any specific constituent in the air sample depends upon the accuracy in the rate of air flow through the air sampling equipment.
One method for measuring air flow is to time the displacement of a film of soap solution between two points in a flowtube. This may be done manually using a calibrated burette and a stop watch. The average flow rate is calculated from the timed rate of rise of the soap film over a defined volume. In conventional commercial units, the soap film is generated as a soap bubble from a soap solution contained in a rubber bulb mounted on the bottom of the burette. Air is directed into the burette through an opening immediately above the level of the soap solution. The rubber bulb is squeezed raising the soap solution to a level to block the inlet opening. Releasing the bulb drops the solution below the inlet opening and a soap bubble is stripped from the solution. Alternatively, the calibrated burette may have an open bottom with the top connected to a suction source. In this configuration, the bubble is generated by raising a container of soap solution to cover the open bottom and lowering it. In each configuration the soap bubble rises through the calibrated burette as a thin film of soap with essentially zero resistance to air flow. The position of the soap film may be be detected visually or electro-optically. Flow measurement is then calculated from a measurement of the transit time of the soap film between two fixed positions along the burette. This calculation may also be automatically performed and displayed on a counter.
The soap film is propelled through the flow tube at a velocity proportional to the flow rate of the air or gas to be measured. The actual velocity varies inversely with the square of the flow tube diameter. Higher velocities introduce turbulence at the surface of the soap solution which aggravates bubble generation. Accordingly, the larger the flow tube diameter the easier it is to generate a soap bubble and conversely the smaller the diameter the more difficult it is to generate a soap bubble. However, for a flowmeter to have practical utility the diameter and length of the flow tube must be limited and preferably miniaturized. When a flow tube with a fixed small diameter and length is used the ability to generate a single coherent soap bubble becomes the major design limitation of the flow meter. This inability to genarate a single soap bubble and the inability to sustain the soap film in the flow tube particularly at higher flow rates of e.g. above 8 liters per minute, has been the major limitation of prior art film flowmeter designs.
Any conventional soap solution composition may be used to form soap bubbles. The inlet air or gas to be measured operates as a driving force which generates the soap film from the soap solution. As the air flow rate is increased, the soap solution becomes agitated resulting in a foam at the surface of the soap solution. Foaming of the soap solution makes it extremely difficult to produce a single coherent soap bubble and is likely to produce inaccurate readings when read electro-optically.
A pulsation in the air flow will cause a sudden change in pressure across the soap film. If the pressure variation is large enough, the soap film or bubble will burst. If the soap film should burst the residue of soap solution must drain out through the flow tube interfering with the subsequently generated soap bubble thereby limiting the interval of time between measurements. It is the present practice in all commercially available flowmeters to forcibly rupture the soap film when it reaches the top of the flow tube so as to force the soap solution to drain backwards through the flowtube. This inherently limits the time interval between successive measurements.